Determination of Vitamin C by Analytical Voltametry.

Determination of Vitamin C by Analytical Voltammetry

wp.55vcvolt 09/09/98

Background

Analytical voltammetry is an electrochemical method in which the changes of electrolysis current are measured when a gradually increasing voltage is applied to the cell. Conditions are adjusted so that the analyte is oxidized or reduced selectively at one of the electrodes in the cell. (See Figure 1.)

In this experiment, a wax-impregnated graphite electrode (WIGE) is used as the indicator electrode. During the electrolysis, ascorbic acid donates electrons to the indicator electrode and the voltammetric experiment exhibits an oxidation (anodic) current step.

The electrochemical process is described in Figure 2 below:

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Figure 1. Analytical Voltammetry

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Figure 2. Electro-oxidation of Ascorbic Acid (Vitamin C).

The quantity of electricity involved in the oxidation peak is directly proportional to the concentration of ascorbic acid. Therefore, an unknown concentration of ascorbic acid can be determined.

Synopsis of the Analytical Problem.

The purpose of this experiment is to determine the concentration of ascorbic acid (vitamin C) in a commercial fruit drink, using the analytical voltammetry technique and a standard addition method. Because the overall composition of the fruit drink is unknown, a blank determination is not possible, and must be estimated.

Special Equipment

Apparatus: Sargent-Welch Model 4001 Polarograph or EG&G-PAR Model 263A

Voltammetric Analyzer; (optional: magnetic stirrer &

stirring bar); ring stand; 3 mini-clamps with insulated claws.

Electrodes: Indicator electrode: Wax-impregnated graphite rod, ~ 6".

Reference electrode: commercial calomel or Ag/AgCl.

Counter Electrode: Spectroscopic graphite rod, ~ 6".

Electrode Polishing: The blunt tip of the WIGE must be polished before running each sample by rubbing it with a circular motion on a piece of filter paper on a flat surface, holding the electrode at the waxed end, perpendicular to the paper.

Preparation of Solutions

(a) Fruit Juice Sample. Turn in a 250 mL volumetric flask to the lab instructor to

obtain an unknown quantity of fruit juice. Dilute this sample to the mark (250.0

mL) with deionized water. The objective of the rest of the experiment will be to

determine the quantity of Vitamin C (ascorbic acid) in this 250 mL volume. The

voltammetric measurements on this solution should be made during the

same lab period in which it was prepared.

(b) Standard Solution of Ascorbic Acid. Weigh out accurately about 0.50 g. of pure

ascorbic acid (F.Wt. = 176.12); record the exact weight, and transfer to a 50 mL

beaker. Dissolve with deionized water and add 1.0 mL of dilute HNO₃. Transfer

to a 100 mL volumetric flask and dilute to volume.

Instrumentation and Measurement Procedures

(a) For Sargent-Welch Model 4001 Polarograph, see Appendix 8.

(b) For EG&G-PAR Model 263A Voltammetric Analyzer, see Appendix 9.

Data Analysis

The standard addition method is used to obtain the ascorbic acid (vitamin C) content of the fruit juice sample. The fundamental analytical relationship is:

Net oxidation current, (i_aa) = kC_aa (1)

where, C_aa is concentration of ascorbic acid and k is a constant which depends on

the area of the electrode, number of electrons, and diffusion rate of ascorbic acid.

The Net oxidation current is obtained by subtracting the background current, i_B, from the total current (see Figure 3). [See Appendix 8 or Appendix 9 for specific Net Current measurement procedures with the Sargent or EG&G instruments, respectively.] For the fruit juice sample, the value of k is not known, because it depends on the overall solution composition, which is unknown; thus, the standard addition method involves adding a known amount of standard analyte (concentration = C_s), and observing the increase in the Net oxidation current (i_a2); this can be repeated to obtain i_a3 (see Figure 3). Because the background current (i_B) is not known, it must be estimated by extrapolating the current observed before the oxidation of ascorbic acid (see dotted line for i_B in Figure 3).

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Figure 3. Standard addition procedure

The increased net current, i_a2, can be related to the added quantity of analyte:

i_a2 = kC_a2 = k[C_aaV_o + C_sDV]/(V_o+DV) (2)

V_o is the initial sample volume in the cell (50.0 mL), and DV is the volume of added standard (1.0mL) . Thus, by dividing Equation (2) by Equation (1), i_a2/i_aa, and solving for C_aa:

C_aa = (i_aaC_sDV)/[i_a2(V_o+DV) - i_aaV_o] (3)

The concentration of ascorbic acid in the original sample, C_aa, can be computed from Equation (3). This computation can be repeated for the second standard addition, where DV is 2.0 mL, and i_a2 is replaced by i_a3. This process can be repeated, but the original matrix becomes changed if too much pure standard solution is added, and the value for k may change. Thus, the volume of added standard must be kept to a "negligible" value.

Questions

1. Why is a standard addition procedure necessary for this determination? Why

not use a calibration curve obtained with standard ascorbic acid solutions?

2. Why is the standard addition volume kept so small?

3. How does the precision of the voltammetric method compare with the titrimetric